It is well known to produce endless lengths of welded steel tubing from strip stock and to continuously galvanize that tubing by providing a zinc coating on the exterior surface as taught, for example, in U.S. Pat. Nos. 3,122,114 and 3,230,615 which are owned by the assignee of this patent application. It is likewise known to continuously apply polymeric coatings to the exterior of such continuously formed tubing, employing various thermoplastic and thermosetting resins, as for example taught in U.S. Pat. Nos. 3,559,280; 3,616,983; 3,667,095 and 3,965,551.
A preferred method of providing the plastic coating on the exterior of the tubing is by passing the formed tubing through a preheating station and then through a powder coating station having a chamber including a particulate fluidized bed of thermoplastic particles or a spray chamber. The coating station is part of an in-line, continuous roll-forming tube mill production line including upstream stations where the steel strip is formed into a tubular configuration and the edges thereof welding together by, for example, an induction welder. The tube mill also includes one or more other stations disposed between the welder and the powder coating station where liquid is applied to the exterior of the tubing. Liquids may be applied for washing and or pickling, rinsing or applying a coating.
Because of the many complicated components which make up the tube mill, the limited service life of certain components and the high speed at which the tubing is produced, the mill must be shutdown not only periodically for component servicing and replacement, but also in the case of a serious malfunction at any one of the stations. The tube mill includes various heaters, dryers and wipers for preventing liquid applied at one of the upstream stations from being carried by the tubing into the coating chamber where the liquid would contaminate the fluidized bed. However, during emergency shutdown situations and during start-up when scrap tubing is generated, the various liquid removal stations may not still (or yet) be operating effectively resulting in potential liquid contamination of the powder coating accumulated in the spray chamber booth due to overspray. If substantial liquid does enter the spray booth, time-consuming clean up of the powder coating station might be required resulting in significant downtime for the production line and loss of production and the cost of the contaminated particulate material.